The present invention relates generally to rechargeable batteries and to electronic circuits for charging rechargeable batteries.
Rechargeable batteries have become quite popular for use in electronic equipment, power tools, portable computers, children's toys, and the like. Virtually every user of rechargeable batteries wants the ability to recharge as quickly as possible. In addition, some users like the convenience of being able to leave a rechargeable battery in the charger at all times, so that a fully charged battery will always be available. To accommodate the former requirements there are a number of quick-charging battery chargers available. To accommodate the latter there are a number of trickle charging battery chargers available.
Whether the battery charger is designed as a quick-charger or as a trickle charger, it is highly desirable that the charger avoid overcharging. Overcharging produces undesired heating and high pressure conditions which can chemically alter and degrade the battery's capacity to hold charge. This problem is particularly prevalent when quick-charging battery chargers are used. Quick-charging battery chargers typically deliver a high charging current which can rapidly overheat the battery unless the charging current is terminated or greatly reduced once the fully charged condition is reached.
Determining precisely when to terminate the high charging current is not simply a matter of sensing when the battery voltage reaches a fully charged level. In most batteries the voltage rises in a nonlinear way as charging current is applied and it is often difficult to accurately sense or predict when the fully charged voltage is reached.
The applicant's assignee has devoted a considerable effort in analyzing the voltage characteristics of rechargeable batteries as charging current is applied. It is now known that the battery voltage increases over time as charging current is applied and that the voltage-time curve exhibits various inflection points where the slope of the first derivative curve of the voltage variation with time curve actually changes from positive to negative or from negative to positive. The Saar et al. U.S. Pat. Nos. 4,388,582 and 4,392,101, assigned to the assignee of the present invention, describe these inflection points in conjunction with a rapid charging system for rechargeable batteries.
Although the inflection point analysis technique described in the Saar et al. patents has been widely successful, there is still room for improvement. Specifically, the Saar et al. technique predicates termination of the charging current based on a falling first derivative of the voltage after a previously detected rising first derivative. The Saar et al. system works well for charging fully discharged batteries. However, a battery that is not fully discharged (e.g. one which starts out already nearly fully charged) often does not exhibit the rising-falling first derivative sequence. When this sequence is not detected it is possible for the charger to continue to supply high charging current to the battery after it is already fully charged. This can cause overtemperature and high pressure conditions in the battery, greatly reducing the battery life and capacity to hold a charge.
The present invention solves the aforementioned problem by utilizing a technique which, at the outset of the charging cycle, performs a state-of-charge test on the battery and from this test determines which control scheme will be used to terminate charging.
More specifically, the state of charge is detected by applying a test current to the battery for a predetermined time (preferably a short period of time) and thereafter sampling the battery voltage at least twice to determine a time rate of change in battery voltage, as the battery responds to the applied test current. In this way, the charging circuit is able to detect where the battery is on the voltage-time charging curve and this information is used to automatically select what voltage inflection conditions to detect in determining when to terminate the charging current.
The technique of the invention is quite effective in preventing overtemperature and high pressure conditions associated with overcharged batteries. This results in increased battery life, faster full charge indication, and greater convenience for the user. For a more complete understanding of the invention, its objects and advantages, reference may be had to the following specification and to the accompanying drawings.